The economics of steel strip and plate production favor the processing of large slabs, sometimes referred to as "jumbo" and "junior jumbo" slabs. These slabs are too large to be used to roll a single strip or plate. Typically, jumbo slabs weigh approximately 30 tons, junior jumbo slabs from 10 to 15 tons and normal or pattern slabs 1 to 2 tons. Pattern slabs are just large enough to produce a single strip or plate following end cropping.
When the product of a pattern slab emerges from the hot rolling mill, it is typically transferred perpendicular to the rolling direction onto a parallel cooling bed where it remains flat until cooled below the brittle temperature range (about 500.degree. F. to 300.degree. F.). Because it is never coiled, the strip or plate cannot suffer the disadvantage of coil memory or uncoiling flaws, such as coil break. On the other hand, the economics of rolling pattern slabs and cooling on a parallel cooling bed are limited. For example, the hot rolling mill must be set up over and over again for each different pattern slab introduced to the mill. The ends of every strip or plate must be cropped reducing the yield from many pattern slabs versus the yield from one or several jumbo or junior jumbo slabs. The capacity of the parallel cooling bed varies depending upon the width of the strip or plate transferred to it due to the uniform placement of the "dogs" that pull the plates over the bed. Unless the strip or plate width is almost an exact multiple of the spacing between the "dogs", the cooling bed is less than 100% covered reducing its throughput.
The current practice for rolling jumbo slabs and junior jumbo slabs is to hot coil the strip or plate and then to place the coils in storage for at least about three days and sometimes weeks before uncoiling, leveling and passing the strip or plate to a cut-to-length line. The three-day cooling period is required because if the coil is unwound while still in the temperature range of 650.degree. F. to 100.degree. F., the strip or plate will suffer from what is known as coil breaks. Coil breaks are visible and undesired metallurgical deformations of the strip that cannot be corrected by further processing.
While the three-day cooling period alleviates or overcomes the problem of coil breaks, it results in another drawback known as coil memory. Once a coil is allowed to cool below 650.degree. F., the strip or plate "remembers" the curved shape of the coil even after leveling and cutting. Leveling (processing through a series of small rolls positioned alternately above and below the strip or plate to alternately pull each surface beyond the yield point) does so in part by balancing the residual stress in the plate around a neutral axis. This results in trapped stresses. On subsequent cutting of the plate or strip, these stresses can produce a shape defect. (This effect is directly related to strip or plate thickness for a given coil diameter.) Also, cooling the coils for from three days to several weeks creates a process inventory that ties up valuable working capital.
The coil memory problem can be avoided by never forming and cooling as a finished coil. A process coiling with flat-pass finishing has been proposed for product rolled from large slabs but not without using a parallel cooling bed and cutting the strip or plate as it emerges from the finishing pass into a length that the width of the cooling table can accommodate.
It is an advantage, according to this invention, to roll jumbo or junior jumbo slabs to strip or plate with in-line cooling (without using a parallel cooling bed) and without experiencing the drawbacks of coil break and coil memory.